The acidities, proton affinities, ionization energies, dissociation energies, and heats-of formation of the hypohalous and hydrohalic acids have been calculated at the G2 level of theory. Where reliable experimental data are available, our results are generally in good agreement but in other cases our predictions serve to fill important gaps. The calculated gas-phase acidities of the hypohalous acids (1507.9 (HOF), 1490.0 (HOCl), 1490.6 (HOBr), and 1487.0 (HOI) kJ mol(-1) at 298 K) agree well with available experimental data and are close to one another (lying within a range of 20.9 kJ mol(-1)), showing that the nature of the halogen has relatively little impact on their acidity. In contrast, the Delta H-acid values for the hydrohalic acids HX increase by 232.9 kJ mol(-1) in going from HF to HI. Hypohalous acids are more acidic than water. In addition, hypofluorous acid is a slightly stronger acid than HF. However, other hypohalous acids are weaker than the hydrohalic acids HX (X = Cl-I). The calculated proton affinities at oxygen (HOX --> H(2)OX(+) : 565.9 (F), 641.9 (Cl), 678.0 (Br), and 724.7 (I) kJ mol(-1) at 298 K) and at the halogen (HOX --> HOXH(+) : 488.7 (F), 581.5 (Cl), 601.0 (Br), and 642.3 (I) kJ mol(-1) at 298 K) are larger than PA(HX) values (484.0 (F), 561.5 (Cl), 584.8 (Br), and 626.0 (I) kJ mol(-1) at 298 K) for all the halogens. The HOXH(+) structures are higher in energy than the O-protonated forms, H(2)OX(+). The ionization energy (IE) values for HOX decrease from HOF (12.71 eV) to HOI (9.89 eV) in a manner parallel to that found for the IE values for HX (X = F-I). The IE(HOX) values are all smaller than the corresponding IE(HX) values, but the IE difference decreases substantially in going from F to I. The G2 heats of formation for the hypohalous acids (-88.3 (HOF), -76.0 (HOCl), -58.3 (HOBr), and -48.9 (HOI) kJ mol(-1) at 298 K) show good agreement with available experimental values.